It is required for telephone systems in the United States that signals received from a telephone subscriber at a central office have a set amplitude. Depending upon the length of the transmission line loop from the central office to the subscriber's location, the current available to the subscriber varies due to the intrinsic impedance of the line itself. This varies the power provided to the subscriber. As the impedance of the line increases, for example, when the subscriber is located a substantial distance from the central station, the subscriber's transmitting circuit must have a higher gain to compensate for the increased telephone line impedance. The increased line impedance also requires increased gain for the subscriber's receiving circuit.
The telephone central office applies a fixed voltage, typically 48 volts, to each subscriber loop and the current provided through the line is therefor a function of the length of the line. This current ranges from approximately 20 to 100 milliamps. Therefore the subscriber's telephone circuit must be able to function with as little as 20 milliamps for a long line but must also be able to operate with as much as 100 milliamps for a short line.
Since the central office does not compensate gain for subscribers and in view of the requirement that the subscriber provide a set amplitude signal to the central office, there exists a need for a circuit to compensate amplifier gain at the subscriber circuit as a function of loop length.